Secondary oil recovery process with incremental injection of surfactant slugs

ABSTRACT

An improved method for recovering oil from underground formations by prior to waterflooding, treating said formations with at least two chemical slugs each containing a synergistic mixture of particular surfactants namely sulfonate-sulfate mixture wherein one slug is compatible with water of high salinity and hardness and the other slug is compatible with the floodwater which optionally can contain a thickener.

lib-b ild OR 3 635i197iib uruwu claws rawnr [151 3,638,728

Hill [451 Feb. 1, 1972 [54] SECONDARY OIL RECOVERY 3,437,140 4/1969Foster etal ..166/273 3,491,834 1/1970 Aheam et al. ...166/273 PROCESSWITH INCREMENTAL 3,508,612 4/1970 Reisberg et al. ..166/274 INJECTION OFSURFACTANT SLUGS [72] Inventor: Harold J. Hill, Houston, Tex.

[7 3] Assignee: Shell Oil Company, New York, N.\.

[22] Filed: Mar. 11, 1970 [21] Appl. No.: 18,734

[52] US. Cl ..l66/273 [51] Int. Cl. ...,E2lb 43/22 [58] Field of Search..166/273, 274

[56] References Cited 7 1 QNFIERI6TEPHEPFLMW 3,288,213 11/1966 King etal ..166/274 3,346,047 10/1967 Townsend et al.. l

Primary Examiner-Ian A. Calvert Attorney-George G. Pritzker and J. H.McCarthy [57] ABSTRACT An improved method for recovering oil fromunderground formations by prior to waterflooding, treating saidformations with at least two chemical slugs each containing asynergistic mixture of particular surfactants namely sulfonate-sulfatemixture wherein one slug is compatible with water of high salinity andhardness and the other slug is compatible with the floodwater whichoptionally can contain a thickener.

6 Claims, No Drawings SECONDARY OIL RECOVERY PROCESS WITH INCREMENTALINJECTION OF SURFAC'IANT SLUGS BACKGROUND OF THE INVENTION Thisinvention relates to the displacement of oil from the pores ofsubterranean, oil-containing reservoirs, and more particularly tospecial surface-active systems to be used with waterfloodingd techniquesto improve the oil displacement efficiency of waterfloods.

Knowledge is widespread in the oil industry that the socalled primaryrecovery techniques, which include natural flow,,gas-lifting,' gasrepressurization and pumping methods, leave substantial quantities ofoil in oil-bearing reservoirs. in addition, there are oil-bearingreservoirs which even though containing large quantities of oil, areincapable of being produced by primary recovery techniques. Recognitionof the large amount of residual oil in many oil-producing reservoirs hasled to the use of the .so-called secondary recovery techniques whichhave as their primary purpose the economical recovery of additionalquantities of the residual oil known to be present in the reservoir.

Probably one of the more common secondary recovering techniques is theso-called waterflooding in which aqueous fluids are injected at onepoint in the reservoir at pressures sufficient to be forced out into thereservoir and toward a spaced production well or wells, thus, in effect,displacing oil from the pores of the reservoir and driving the oil aheadof the water front.

However, waterflooding is only advantageous when the cost of injectingwater and necessary chemical modifiers is less than the value of the oilrecovered, Therefore, the displacement efficiency of waterfloods hasbeen thedetermining factor or whether such a technique will be used.

Generally, the difficulty with waterfloods is that the small pores andcapillaries of the reservoir contain hydrocarbons (oil and/or oil andgas) which are generally water immiscible. The existence of highinterfacial tensions between the boundary of the water and hydrocarbonsseriously impedes the ability of the water to displace oil trapped inthe reservoir by capillarity.

Since in many oil reservoirs the oil tends to be trapped within pores ofthe interconnected capillary system of the rock formations bycapillarity, merely forcing water therethrough will not displace much ofthis trapped oil. However, a reduction in the interfacial tensionbetween-the water and the oil will tend to increase the amount of oilthat will be displaced by the water. Thus, various aqueous surfactantsystems have been proposed for use in waterflooding processes forrecovering the oil. In many such prior systems the interfacial tensionbetween the oil and water is reduced from a characteristic value in theorder of 50 dynes per centimeter to a value 'of from about 1 to 10 dynesper centimeter. if the interfacial tension is reduced to only about I to10 dynes per centimeter, the increase in the amount of oil that can berecovered is not generally significant. Thus, the cost of obtaining thereduction in the interfacial tension is apt to be more than the value ofthe increased amount of oil that is recovered in many cases.

Many liquid systems containing surfactants have been used to aid in oilrecovery and among the most promising of such systems are thosedescribed in U.S. Pat. Nos. 3,275,075; 3,324,944; 3,330,344; 3,348,611;3,366,174; 3,469,630; 3,477,508; 3,477,511; 3,478,823; 3,480,080 andNetherlands Pat. No. 6,812,398, in which improved and effective oilrecovery is achieved by a waterflood in which a frontal portion is aliquid containing surfactants such as surfactant micelles that arecombined with molecules of an amphiphilic organic compound of low watersolubility. Generally, in these systems, the concentration of thesurfactant in the aqueous solution exceeds the critical concentrationfor micelle formation and the amphiphilic material swells or becomesassociated with the surfactant micelles and causes the aqueous solutionto exhibit little or no interfacial tension against the oil.

Although the surfactant systems described in the abovecited patents aregenerally effective, their beneficial effects tend to be reduced whenpolyvalent metal ions are present in the earth formation. Such ions tendto cause either the precipitation of the surfactant or the generation ofviscous emulsions and the plugging of the pores of the earth formation.in additiomthe surfactant materials used in such prior systems tend tobe absorbed to an extent that is disadvantageous on the surfaces ofoil-containing earth formations.

Although the pretreatment chemical process as described in the U.S. Pat.application, Ser. No. 752,882, filed Aug. 15, I968 and now U.S. Pat. No.3,508,612, is superior to the prior art as illustrated by the referencescited above thereby resulting in improved secondary oil recovery,problems arise where formation water or injected water is of highsalinity and hardness causing plugging and emulsion formation, saidplugging and emulsion formation adversely affects relative mobilities,resulting in a decreased sweep efficiency of the recovery system therebyresulting in inefficient oil recovery.

SUMMARY OF THE INVENTION It has now been found that the abovedeficiencies of chemical slug treatment as described in the above-citedart can be successfully obviated and an increased tolerance of chemicalslugs for high salinity and hardness of formation and/or injected salinewater and improved mobility and sweep efficiency can be obtained byincremental or staged injection of the chemical slugs so that one ofsaid slugs has a high compatibility for hard and/or saline formationwater and the other slug has a high compatibility for the floodwaterwhich may be fresh water which may contain a thickener, pusher or otherviscosity-increasing agent, resulting in improved mobility, permeabilitycontrol and oil recovery.

The staged or incremental injection of at least two chemical slugs ofthe present invention, each of which generally comprise aqueoussolutions containing different mixtures of dissimilar surfactants, ofwhich preferably at least one is an alkali metal or ammonium or aminesalt of an organic sulfonate, and the other one is an alkali metal orammonium or amine salt of a sulfated oxyalkylated organic material. Thefirst slug should preferably contain relatively high concentrations ofthe additive polyoxyalkylated organic sulfate so as to be compatiblewith saline and formation water and the second slug containingessentially the same chemical additive mixture but in low concentrationsso as to be compatible with fresh water or the drive water which maycontain a thickener or viscosity-increasing agent. Staged or incrementalinjection of these properly designed chemical slugs achieves improvementin mobility and sweep efficiency at minimum concentration of thickenersin the drive water or other places in the recovery process fluids. Theuse of builders or sacrificial additives such as tripolyphosphate,organic phosphates, sodium carbonates which minimize adsorption ofsurfactants onto the formation surfaces, is also minimized oreliminated.

In essence, the present invention improves the compatibility of a mixedsulfonate-sulfate surfactant system relative to a situation in which thesalinity of the water in the reservoir is different from that of thedrive water. The improvement is effected by dividing the system into atleast two portions and adjusting the ratio of sulfonate-to-sulfate byweight ofeach portion to enhance its individual compatibility with thewater that is contacted by it. Where the salinity of the water isrelatively high, the ratio of sulfonate to sulfate is low, and viceversa.

The present invention is an improvement over the system described incopending application Ser. No. 752,882, filed Aug. 15, I968, (U.S. Pat.No. 3,508,612) since such a singleinjection system has been found tohave a tendency to be either incompatible or inactive (relative tooildisplacement efficiency) where, for example, the formation water ishighly saline and the drive water is relatively fresh. Thus, in asingleinjection system it has been found that if the sulfonate-tosulfateratio is low enough to ensure compatibility with a highly salineformation water, the system tends to lose activity as it becomes dilutedby a relatively fresh drive water. In the present two-component system,the overall behavior of a composite system that has a front portion inwhich the ratio is relativcly low and a rear portion in which the ratiois relatively high provides substantially all of the advantageousproperties, such as adequate activity and low adsorption, thatare'characteristic of the systems of the type described in the copendingapplication, but avoids the disadvantage of either poor compatibilitywith the saline water ahead or a loss of activity due to dilution by thefresh water behind.

DESCRlPTlON OF THE PREFERRED EMBODlMENT The organic sulfonate useful asone of the additives in the solution slugs can be substantially anysurfactant salt of an organic sulfonate provided it possessessurfactant-amphiphile properties generated by the oil solubility versuswater solubilit y spectrum of the sulfonate. In such surfactantsulfonates the oil-soluble (generally higher molecular weight)constituents serve as amphiphiles and are solubilized in the aqueoussystem by the primarily water-soluble (generally lower molecular weight)sulfonates. Anionic sulfonate surfactants of this type can beillustrated by metal or ammonium salts of sulfonate surfactants, e.g.,alkali metal, ammonium or polyvalent metal salts of sulfonated petroleumhydrocarbons such as alkylated naphthalene sulfonates, alkylated benzenesulfonates; or the sulfocarboxylate salts, and the like; Preferredsulfonates are the alkali metal (Na, K, Li) salts of petroleumsulfonates such as C alkyl aryl sulfonates, alkylated benzene sulfonatesand the like. Materials of this type are sold commercially under varioustrade names such as petroleum sulfonates sold by Bray Chemical Companyor the Bryton Chemical Company as Bryton sulfonate, F, 430, 467,500, orthe Sonnebom Chemical Company as Petronates, or the Socony MobileCompany as "Promor" sulfonates of theSS-6, SS-20 series, AmericanCyanamids Aerosol T which is Na dioctyl sulfosuccinate and the like. Apreferred sulfonate mixture is a sodium salt of a petroleum sulfonate inthe molecular weight range of 350-420 and a sodium salt of a petroleumsulfonate in the molecular weight range of 420-580 or a sodium salt of amixture of petroleum sulfonates having an average molecular weight inthe order of about 430-470.

The sulfated polyoxyalkylated organic material surfactant can beprepared by suitable means such as sulfating with chlorosulfonic acid,sulfur trioxide, sulfonic acid, oleum or sulfuric acid in a suitablesolvent, a surface-active water-soluble oxyalkylated organic materialhaving from about 8 to about 20 carbon atoms and represented by thegeneral formula (1) represented by where R is an a y r real or orgamcmoiety of 8 to 20 and preferably 10-15 carbon atoms, R and R" aredissimilar alkyl radicals or groups from l-6 carbon atoms and preferablyare -C,l*l, and -C H radicals, respectively, x and z are positiveintegers of at least 1 and y can be zero or a positive integer as x or2. Preferred materials comprise oxylated alcoholic compounds such asoxylalkylated alkanols which can be represented by the general formula(II) represented by where R is an alkyl radical of -15 carbon atoms, thealcohol used in the ethoxylation being a mixture of alcohols of 10-15carbon atoms. The ethoxylation of the alcohols can be accomplished bymeans well known in the art. The ethoxylation of primary alcohols toform products represented by formula (11) can be achieved using a strongbase of Lewis acid catalysts such as NaOl-i, BF or SnCl,. Oxyaikylatedprimary alcohols represented by formulas (l) and (11) can be prepared bythe method described in U.S. Pat. Nos. 3,036,130 or 3,101,574 or asdescribed in copending U.S. Pat. application to Tsatsos et al. Ser. No.661,546,filed Aug. 18, 1967.

and 3,370,649.

TABLE 1 Chemical Recovery Systems One-Step Process Staged or IncrementalProcess Parent System" Sulfonate-DEODOL 25-38 System Slug A Slug 8Component: Na petroleum sulfonate (NaSO,R )meqtlg. 0.03 0.034 0.034 Natrlpoly phosphate (Na P O dwt. i: 0.5 Na chloride (NaCl wt. I: 0.9

Polyacrylamide (Pusher 520) wt. 1v 0.04 Polyacrylamide (Pusher 700) wt.'i: 0.01 0.02 Na sulfate C alcohol containing three ethoxy groups(NEODOL 25-38) wt. 1: 0.6 0.2 BPW' wt% 8.0 g 10.0 LMW' wt. 96 -96.S -89]-87.8 Compatability' with:

BPW wt. 18-25 50 15 LMW wt. i: l0-l5 25 50 Viscosity (95' F.) cp. at 46sec. 1.7 1.5 2.6 Screen Factor 8.9 7.7 10.3

The sulfation of such a surface-active polyethoxylated material can beaccomplished by reacting neat or in the presence of a solvent using asthe sulfating agent any of the materials mentioned previously andpreferably sulfur trioxide. chlorosulfonic acid, or sulfuric acid. Thus,any of the ethoxylated alcohols can be sulfated by dissolving thealcohol in a solvent such as ethyl ether and adding dropwisechlorosulfonic acid keeping the temperature at 0-l0 C. The HCl formed isremoved by bubbling N, through the solution and the solution neutralizedby adding gradually an aqueous methanol solution containing theoreticalamounts of base. The solvent is then removed if desired.

Sulfates of ethoxylated primary alcohols are commercially available fromShell Chemical Company under the trade name Neodol 23-3A having theformula C .,,O(CH,CH O) SO N H, and Neodol 25-38 C, 0(C1-l CH O);,SO5Na.

Other anionic sulfated ethoxylated alcohols are available from UnionCarbide under the trade name Tergitol S such as Tergitol Anionic l4-S-3A(ammonium salt) or 15-2-30 (sodium salt) having the formula:

O-(CHzCH20)3-SO3X where X is NH, or Na.

Salts of sulfated polyethoxylated alkyl phenols also can be used incombination with organic sulfonates.

The effectiveness of waterflooding an oil-containing undergroundformation with staged or incremental slugs of aqueous slugs containingthe additive mixture of the present invention, each slug containingdifferent concentrations of the additive mixture; namely, sulfonatesurfactant mixed with sulfated polyethoxylated organic surfactant, e.g.,Na petroleum sulfonate (av. MW 430 or 470) and Na sulfate C alcohol (3-9E0) to aid in oil recovery is illustrated by the data presented intables 1 and 11. The waterflooding fluid can be water or thickened waterin which the thickening agent is a water-soluble salt of a hydrolyzedpolyacrylamide of high molecular weight (1-10 million) and availablecommercially from Dow Chemical Company under the trade name Pusher 520,Pusher 700" or ET-601 or Separan NPlO, NP20, AP30, AP273, P62 or Mydel550 as described in U.S. Pat. Nos. 3,002,960; 3,022,279; 3,039,529;3,074,481; 3,369,977

"Benton. lll. field produced water; total dissolved solids 7.5 percent.

Lake Moses water, lll.; total dissolved solids -0.02 percent.

'Compatibilitywhen system is blended with stated amounts or less of BPWor LMW, no precipitation of components occurs and the resulting blend isstill capable of displacing residual oil; efl'lciency is reduced becauseof dilution. Adsorption is increased in BPW blends.

"Parent System: Ser. No. 750,882, filed Aug. 15,1968(U.S. Pat. No.3.508.612), one-step chemical injection.

Table 1 indicates that the stage or increment chemical slug injectionsystem significantly increased compatibility for high salinity, highcalcium content formation water. It is completely compatible with 25percent produced water. In addition, it is still capable of oil recoveryin 50-50 mixtures with formation water. At these concentrations offormation water the parent system would precipitate sulfonate, or, inthe presence of oil, transfer sulfonate to the oil and form viscousemulsions.

Adsorption of surfactant from the stage system may be 25-50 percenthigher than adsorption from the parent system (deletion of sacrificialagent). Total surfactant concentration is 41 percent higher in stage Aand 23 percent higher in stage B; providing an average of 32 percentmore surfactant in each barrel of slug which compensates for thepossible increase in adsorption.

Oil recovery capability of the present staged or increment process hasbeen compared with that of the one-step parent process in Berea cores upto 8 feet in length (tests L-4, L-5, table 11) and total oil recoveredby the present process is better than the process of the patentinvention. The increased compatibility of the staged surfactant slugsmakes possible elimination of the pretlood which was required for theparent process.

TABLE II One-Step Process Staged or Increment Process Parent SystemSuIfonate-NEODOL ZS-JS System Core data Length (er-n.) 486 244 74.8

Permeability (md.) 742 717 769 Porosity V 22.5 22.2 22.2 Process datafresh water 0.05 0.00 0.00

Surfactant slug,

total (V,) 0.25 0.25 0.30

Polymer slug (V,) 1.0 0.75 0.75

Fresh water (V,,) 1.2 0.48 0.75 Performance data 5,, at waterflood 0.380.35 0.38

S, at 0.5 V,

process fluids 0.27 0.27 0.31 g S. at 1.0 V,I

S, at 1.25 V,

process fluids 0.08 0.05 0.07

S, at 1.5 V,

process fluids 0.08 0.04 0.04 S S, (extraction) 0.06 0.02 0.00

Oil breakthrough Surfactant breakthrough 0.90 0.97 1L08 Oil cut 0.290.30 31 Fraction of produced fluids. averaged from oil breakthrough to1.25 V, injection process fluids.

To summarize the staged or incremental injection of the chemical systemproduces better total oil recovery with less total injection (nopreflood) because of improved compatibility with both produced water andpolymer drive which prov1des 1nsurance agamst excessrve mrxmg andprocess breaklowed by water-thickening materials dissolved in aqueousliquids. The water-thickening materials may comprise natural orsynthetic polymeric materials such as water-soluble gums and/0r polymerssuch as partially hydrolyzed polyacrylamides such as Dow ChemicalPushers 520 and 700.

The first injected surfactant' slug should contain the organic sulfonateand the sulfated ethoxylated surfactant in a concentration ratio suchthat (l) the resulting slug is interfacially active (lowers interfacialtension adequately) in the presence of oil in the formation to beflooded and (2) the slug is compatible with the water in the formationto be flooded. While the organic sulfonate concentration in the secondslug may be either equal to, greater than, or less than theconcentration in the first injectedslug, the ratio of concentrations oforganic sulfonate to ethoxylated sulfate surfactant in the second slugshould be such that the resulting slug remains interfacially activewhile the slug is being diluted with the drive liquid.

It is understood that various changed in the details described toexplain the invention can be made by persons skilled in the art withinthe scope of the invention as expressed in the appended claims.

1 claim as my invention:

1. A method of producing oil from oil-containing reservoir formationcontaining formation or saline-hard water comprising the steps of:

a. injecting into said formation a slug of aqueous solution containing amixture of dissimilar surfactants at least one of which is a salt of anorganic sulfonate and at least one of which is a salt of a sulfatedoxyalkylated alcohol surfactant, the cationic portion of each of saidanionic salts being selected from the group consisting of alkali metal,ammonium and amine, in a ratio by weight of the sulfonate to theoxyalkylated surfactant, which is compatible with the salinity of theformation water;

b. injecting into said formation a slug of aqueous solution containing amixture of dissimilar anionic surfactants at least one of which is asalt of an organic sulfonate and at least one of which is a salt ofsulfated oxyalkylated alcohol, the cationic portion of each of saidsalts being selected from the group consisting of alkali metal, ammoniumand amine, in a ratio by weight of the sulfonate to the oxyalkylatedsurfactant which is compatible with the water drive of (c) and whereinthe ratio of the corresponding additives in slurry (b) is higher than inslurry (a); and

. waterilooding the formation with water having a salinity less thanthat of the formation water which has been treated with slugs (a) and(b) to effect oil recovery.

2. The method of claim 1 wherein the sulfonate is a petroleum sulfonateand the oxyalkylated surfactant is a polyethoxylated alcohol sulfate.

3. The method of claim 2 where in the petroleum sulfonate is naoil-soluble Na petroleum sulfonate and the oxyalkylated surfactant is asulfated polyethoxylated primary alcohol.

4. The method of claim 3 wherein the sulfated alcohol has from three tosix ethoxy groups in the molecule.

5. The method of claim 4 wherein he drive water is water containing athickener.

6. The method of claim 5 wherein the thickener is a polyacrylamide.

2. The method of claim 1 wherein the sulfonate is a petroleum sulfonateand the oxyalkylated surfactant is a polyethoxylated alcohol sulfate. 3.The method of claim 2 where in the petroleum sulfonate is na oil-solubleNa petroleum sulfonate and the oxyalkylated surfactant is a sulfatedpolyethoxylated primary alcohol.
 4. The method of claim 3 wherein thesulfated alcohol has from three to six ethoxy groups in the molecule. 5.The method of claim 4 wherein he drive water is water containing athickener.
 6. The method of claim 5 wherein the thickener is apolyacrylamide.